Nanoparticles of various semiconductors can be synthesized by means of a well known colloidal organic process (see “Efficient solution-processed infrared photovoltaic cells,” Applied Physics Letters, 90, 183113) by deposition from a gas phase or by sputtering. The typical size of a nanoparticle (required for band gap tuning and quantum confinement) ranges from 2 to 8 nm. Nanoparticles may have different shapes, such as quantum dots, wires, rods, crystals, etc. Nanoparticles suitable for photovoltaic (PV) application are exemplified by Si, Ge, CdTe, CdSe, CdS, InP, TiOx, InAs, PbS, PbSe, HgTe, CuInSe2, CuInGaSe2, etc. (see “Advanced inorganic materials for photovoltaics,” MRS Bulletin, Vol. 32, pp. 211 to 214).
Typically, in manufacture of a PV device, a nanoparticle-containing solution is deposited onto a precoated transparent substrate (e.g., glass) and consequently is converted into a very thin film (30 to 200 nm) of a selected type by the spin-casting procedure. Nanoparticles also can be embedded in some photoactive organic films (e.g., polymers) to form a “hybrid” hetero-junction PV device or a bilayer structure to form a bulk hetero junction (see “Semiconductor-nanocrystal/conjugated polymer thin films,” U.S. Patent Application Publication, US2005/0133087, inventor Alivisatos, A. Paul, et al, which discloses manufacture of thin films comprising inorganic semiconductor nanocrystals dispersed in semiconducting polymers in high loading amounts).
In addition to the potential simplicity and low processing cost of nanoparticle-based films, such films possess some unique material properties highly desirable for PV applications. Such properties relate to better ability of nanoparticles to use incoming radiation energy, particularly in the IR portion of the spectrum, as compared with bulk and TF counterparts. The aforementioned properties contribute to the mentioned capability of nanoparticle-based materials (for further explanation see “The photoconversion mechanism of excitonic solar cells,” MRS Bulletin, Vol. 30, pp. 20 to 22 and “Solar cells based on quantum dots: multiple exciton generation and intermediate bands,” MRS Bulletin, Vol. 32, pp. 236 to 240).
It should be noted that nanoparticle “patterning” mentioned in some references, such as U.S. Pat. No. 6,696,107 issued in 1994 to Derek A. Eastman and U.S. Patent Application Publication US2006/093749, typically relates to device layout formation along the active surface of a device (i.e., horizontal patterning) rather than cross-section patterning across active nanoparticle layers (i.e., vertical patterning). Other types of nanoparticle-based films can be formed in the complimentary layer that has a junction interface with the first layer, e.g., as in the reference “Air-stable all-inorganic nanocrystal solar cells processed from solution,” Science, Vol. 310, pp. 462 to 465. Although the size of a nanoparticle in some of the aforementioned film can be well controlled (see Patent Application Publication WO2008/136882 and U.S. Patent Application Publication 2007/111697), all aforementioned methods of depositing nanoparticle-based layers are incapable of providing nanoparticle-structured films that consist of nanoparticle layers positioned in a predefined order of size or type of material. The aforementioned methods do not provide for proper control of nanoparticle layer structure across film with regard to type of material or size.
In other words, known methods of forming nanocomposite films by depositing and processing nanoparticle-based layers are fundamentally limited in producing predefined nanoparticle spatial arrangements of size or type of material. Consequently, PV devices made of such nanocomposite films typically exhibit low carrier transport and collection efficiency (i.e., internal quantum efficiency), incomplete spectral coverage, limited Voc, etc. Some advanced nanocomposite structures for PV applications are described in “Nanostructured Organic-inorganic Hybrid Solar Cells,” MRS Bulletin, Vol. 34, pp. 95 to 100. It should be noted, however, that none of the proposed structures present a nanostructured film that comprises a sequence of nanoparticle layers arranged in the order of their size or type of material.
U.S. Patent Application Publication 20080142075A1 published in 2008 (inventor D. Reddy, et al) describes nanoparticle-based films and related PV devices that may have nanoparticle layers formed on a substrate in order of nanoparticle size (nanostructured film). Certain PV devices, such as solar cells, are described as using nanoparticle-based film of the aforementioned type as one of the PV-active layers. According to the aforementioned invention, such a PV-device will have an electropotential gradient (i.e., an electric field) that results from a nanoparticle size gradient and a related shift in energy band, thus significantly improving the drift component of photo-generated carriers. That, in turn, will result in enhanced carrier transport and collection efficiency (i.e., internal quantum efficiency) of the device. Also, based on the photo-conversion mechanism in nanoparticle-based materials as described in “The photoconversion mechanism of excitonic solar cells,” MRS Bulletin, Vol. 30, pp. 20 to 22, PV devices made of such materials are expected to provide higher Voc.
U.S. Patent Application Publication 20080108122 published in 2008 (inventor B. K. Paul, et al) discloses embodiments of microchemical nanofactories that could be used to fabricate a variety of tailored gradient structures from nano-, micro-, and macroscale particles in which the particles are arranged in sequential layers and vary in size or density, or both.
However, known methods for forming sequential layers of PV nanostructures with gradual change in nanoparticle size or type of material in the direction away from the substrate requires highly complex and lengthy deposition and control processes, which is not a cost-effective solution. Furthermore, it is possible that application of any layer may partially or entirely damage the preceding nanoparticle layer and that the suggested method will not be justifiable for coating surfaces of large areas.
Also known in the art are methods for forming intermetallic nanoparticles by evaporating a liquid component of a nanoparticle-containing solution, which is achieved, e.g., by irradiating such a solution with a high-energy beam.
U.S. Pat. No. 6,368,406 issued in 2002 to S. Deevi, et al, discloses a method of forming intermetallic nanoparticles by subjecting a starting material to laser energy so as to form a vapor and condensing the vapor so as to form intermetallic nanoparticles. The starting material can be a mixture of pure elements or an alloy of two or more elements. The nanoparticles can be provided with a narrow size distribution, with an average particle size of 2 to 100 nm, preferably 2 to 9 nm. The nanoparticles can be formed in a vacuum chamber wherein a temperature gradient is provided. The atmosphere in the chamber can be an inert atmosphere, such as argon, or a reactive atmosphere, such as isobutene or oxygen. An electric field can be used to form filaments of the nanoparticles.
U.S. Pat. No. 5,770,126 issued in 1998 to J. Singh, et al, discloses a process and apparatus for producing nanoscale particles using the interaction between a laser beam and a liquid precursor solution. According to one embodiment, a solid substrate is used during laser-liquid interaction. The laser beam is directed at the solid substrate, which is immersed in the liquid precursor solution, and rotates.
However, the patents that describe methods and apparatuses for separation of nanoparticles from solutions do not teach any technique for arranging nanoparticles into specifically structured layers, in particular into layers composed of nanoparticles structured in terms of their sizes or masses.